Add code_mode experimental feature (#13418)

A much narrower and more isolated (no node features) version of js_repl

codex-rs/core/src/features.rs

@@ -83,6 +83,8 @@ pub enum Feature {
     // Experimental
     /// Enable JavaScript REPL tools backed by a persistent Node kernel.
     JsRepl,
+    /// Enable a minimal JavaScript mode backed by Node's built-in vm runtime.
+    CodeMode,
     /// Only expose js_repl tools directly to the model.
     JsReplToolsOnly,
     /// Use the single unified PTY-backed exec tool.
@@ -510,6 +512,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         },
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CodeMode,
+        key: "code_mode",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::JsReplToolsOnly,
         key: "js_repl_tools_only",